1. Field of the Invention
The present invention relates to fixed wireless communication networks and more particularly to a method of frequency reuse in such a communications network.
2. Description of the Related Art
As depicted in FIG. 1, a Local Multi-point Distribution System (LMDS) 8 is a fixed access wireless system comprised of a collection of base transceiver stations (BTS) 10 which broadcast and receive signals from a large number of terminal radio stations (TS) 12. The terminal stations are fixed and equipped with directional antennas (not shown), each of which are oriented toward a serving BTS. The base transceiver stations 10 can accommodate omni-directional transmission in which case they are equipped with omni-directional antennas, or they can be sectorized in which case they are equipped with directional sector antennas. A specified base transceiver station 10 and its associated terminal radio stations 12 comprise a xe2x80x9ccellxe2x80x9d 14 the basic building block in a wireless network. The cells 14 are theoretically considered to be circular in shape. The base transceiver stations 10 are each connected to a backbone network 16, which may be a computer network, a cable television network, a public telephone network or the like. Connection to backbone network 16 may be accomplished using copper wire, fibre optics, wireless transmission or a combination of these communication means.
An LMDS 8 typically operates in the frequency range of 24.0 GHz to 42.0 GHz. In general, LMDS system architectures generally differ in terms of cell size, modulation format and BTS antenna type. Other system design parameters include antenna patterns, antenna heights, antenna pointing, cell spacing, frequency reuse plan, polarization reuse plan and link budget. For example, a two-way multiple access LMDS system described by Texas Instruments (TI) to the FCC Negotiated Rulemaking Committee (NRMC) on the LMDS/FSS 28 GHz band, July-September 1994, utilizes 52 Mbps Quadrature Phase Shift Keying (QPSK) and four directional sector antennas at each BTS to provide omni-directional cell coverage with a nominal cell radius of 5 km. Typically multiple TSs within a cell are serviced by their associated BTS in accordance with time and frequency division multiplexing techniques. In operation, a given BTS broadcasts on a number of assigned frequencies with each TS antenna programmed to receive on one designated frequency.
A broadband fixed wireless network as described above provides the physical infrastructure to provide wireless access to services ranging from one-way video distribution and telephony to fully-interactive switched broadband multimedia applications. Various techniques are employed to maximize the amount of traffic which can be carried in the network. As noted above, efficient modulation schemes from QPSK to Quadrature Amplitude Modulation (e.g. 64 QAM) are used to increase the frequency efficiency of the system. Additionally, due to the limited frequency range available for operation of an LMDS, frequency reuse is employed for re-assigning available frequencies between cells in a fixed wireless network. Typically, a network manager utilizing a programmed general purpose computer would determine the manner in which frequencies will be allotted between the cells 14 in an LMDS 8.
Frequency reuse is a method of optimizing spectrum usage, enhancing channel capacity, and reducing interference. It will be appreciated by those in the art, interference can also be reduced using other techniques such as space separation of transmitting equipment, time separation of broadcasts and signal polarization. Frequency reuse involves channel numbering, channel grouping into subsets, and assigning particular channels to particular cells. A plurality of cells are then associated together into clusters and utilize all of the assigned frequency channels in a prescribed manner. Groups of clusters are then used to provide coverage over a defined geographic area and the frequency channels allocated to one cluster are reused in other clusters. The scheme for recycling or reassigning the frequency channels throughout the coverage area is referred to as a reuse plan. The distance between a first cell within a cluster using a particular frequency channel and a second cell using the same frequency channel is further known as a reuse distance. The principal objective of such a reuse scheme is to ensure adequate channel isolation to reduce channel interference while maintaining a high channel capacity.
It will be understood by those skilled in the art that the term decibel which will be used throughout the description, is a common unit used in relation to radio frequency transmission to denote relative differences in signal strength and is expressed as the base 10 logarithm of the ratio of the powers of two signals i.e. dB=10 log (P1/P2). Logarithms are useful as the unit if measure because signal power tends to span several orders of magnitude.
As explained above, the reuse of the same frequency channels by a number of different cells implies that cells may suffer from co-channel interferences. Depending on the operating frequency band, the terminal station directional antenna can have a high gain in the range of 25 to 45 dbi. Therefore, the directional antenna will reject most of the interfering radio signals from nearby BTSs, except those signals arriving at the TS from distant BTSs lying in the same direction as the BTS serving the TS. It is generally desirable for the received strength of the serving carrier (C) within each cell to be higher than the total co-channel interference level (I). As a result, the higher the carrier to interference (C/I) ratio, the better the data transmission quality. A higher C/I value is obtained partly by controlling the channel reuse distance. The larger the reuse distance between adjacent cells utilizing the same frequency channels, the lesser the co-channel interferences created between those cells. Since the C/I ratio is normally dictated by, among other things, the equipment used in the wireless network (i.e. its ability to discriminate a useful signal), in order to maximize frequency reuse, the minimum acceptable reuse distance is identified for a stated C/I and the available frequencies are distributed between cells accordingly. A number of other physical factors can also affect C/I in wireless networks e.g. buildings, geography, antenna radiation patterns, and transmitting power.
It will also be appreciated by those in the art that there is a trade-off between modulation schemes and frequency reuse plans. The higher and more efficient the modulation scheme, the higher the minimum required carrier to interference (C/I) level, which forces a reduced frequency reuse factor (the ratio between the amount of bandwidth used at each cell and the total frequency bandwidth available). For example, if 16 QAM is used instead of QPSK, the frequency planner would have to consider BTSs which are further away since a higher C/I ratio is required to meet the QAM specification (i.e. in theory 4 bits/s/Hz). The C/I ratio is related to the frequency reuse plan (N/F) where N indicates the number of cells included within a single cluster and F indicates the number of frequency groups. For example, the C/I ratio is directly related to the following equation:
DR=(3xc3x97F)xc2xdxc3x97R
where: DR is the reuse distance; F is the number of frequency groups; R is the radius of a cell.
Accordingly, the larger the F value, the greater the reuse distance. However, it is not always possible to use a larger F value to increase the C/I ratio. Since the total number of available frequency channels (T) is generally fixed within a wireless network, if there are F groups, then each group will contain T/F channels. As a result, a higher number of frequency groups (F) would result in fewer channels per cell and lesser transmission capacity.
In a mobile cellular radio system, capacity is not a major issue when the system initially goes into operation. Therefore, in order to achieve a high C/I value, a high frequency reuse plan (N/F), such as 9/27, is initially used. However, as the capacity increases, the mobile cellular radio system has to resort to a lower frequency reuse plan, such as a 7/21 or 4/12, to allocate more frequency channels per cell.
A prior art method of symmetrical frequency reuse in a mobile cellular radio system begins with two integers, i and j, that are referred to as shift parameters. The frequency plan is established by starting with a reference cell and moving over i cells along the chain of cells. After reaching the ith cell, a counter-clockwise turn of 60xc2x0 is made and another move of j cells is made. The jth cell can safely be a co-channel cell. The frequency plan can also be established by moving j cells before turning i cells or by turning 60xc2x0 clockwise.
After all the possible co-channel cells of the initial cell are laid out, another reference cell is chosen and the procedure is repeated. This entire procedure is repeated as often as necessary to establish the frequency reuse plan over the entire wireless network.
The cells thus established by the above procedure form a reuse pattern of i2+ij+j2 cells. The number of cells in this reuse pattern is a predominant concern of the cellular industry since this number determines how many different channel groups can be formed out of the frequency spectrum allocated to the network. A low number of cells in a reuse pattern means more channel groups can be formed and more users accommodated.
Although the above frequency reuse scheme works adequately, it is not appropriate for a Local Multi-point Distribution System (LMDS). The frequency reuse schemes developed for mobile cellular networks have been created for radio access systems where subscriber terminal radio stations are equipped with non-directive antennas. In fixed access wireless networks like an LMDS, the terminal radio stations are equipped with highly directive antenna, which reject most of the intra-system interfering signals coming from other than the serving base station i.e. there is generally less interference arising which offers different possibilities from a frequency reuse perspective. Frequency reuse schemes need to be developed which exploit the directivity of the terminal station antennas.
The present invention serves to overcome the deficiencies of the prior art by providing a method of assigning frequencies to a fixed wireless network, specifically an LMDS. By exploiting the directivity of the fixed terminal stations, the method serves to increase overall traffic capacity in an LMDS where a specified number of frequencies have been assigned. In a wireless network comprising ixc3x97j cells, each cell is divided into an even number of at least four sectors. Within each sector there exist interference zones, the number and magnitude of which are a function of the number and location of cells in the network and the minimum required C/I in the network. Given a desired carrier to interference ratio, the cells are grouped into clusters, the cluster size defining the interference boundary for a given cell in the cluster. A high frequency reuse factor is achieved by controlling the frequency reuse assignments to interference zones within each sector of the cells within a cluster.
In one aspect of the invention there is provided in a fixed wireless network utilizing an assigned group of frequency channels and comprising ixc3x97j cells, wherein each of the cells is divided into an even number of at least four sectors, and wherein each of the cells includes a base station and a plurality of terminal stations, a method of frequency reuse comprising the steps of: receiving input data including: (i) a desired carrier to interference ratio; (ii) the number of cells in the network; and (iii) the number of frequency channels available; calculating a reuse distance based on the desired carrier to interference ratio; calculating a cluster size based on the calculated reuse distance; determining the position and number of interference zones within each of the sectors, wherein the interference zones are a function of the cluster size; assigning frequency channels to the interference zones; determining which of the allotted frequency channels cannot be used within a specified one of the cells; determining which of the allotted frequency channels cannot be used within a specified one of the sectors; and assigning any remaining ones of the allotted frequency channels to one or more neutral zones extending between the interference zones.
In another aspect of the present invention, there is provided a system for facilitating data transfer comprising: a backbone network; a fixed wireless network utilizing an assigned group of frequency channels and comprising ixc3x97j cells, wherein each of the cells is divided into an even number of at least four sectors, and wherein each of the cells includes a base station in radio frequency contact with a plurality of terminal stations located in the cell, the base station also communicating with the backbone network; and means for maximizing the data traffic which can be processed through the fixed wireless network, wherein said maximizing means carries out at least a method of frequency reuse comprising the steps of: receiving input data including: (i) a desired carrier to interference ratio; (ii) the number of cells in the network; and (iii) the number of frequency channels available; calculating a reuse distance based on the desired carrier to interference ratio; calculating a cluster size based on the calculated reuse distance; determining the position and number of interference zones within each of the sectors, wherein the interference zones are a function of the cluster size; assigning frequency channels to the interference zones; determining which of the allotted frequency channels cannot be used within a specified one of the cells; determining which of the allotted frequency channels cannot be used within a specified one of the sectors; and assigning any remaining ones of the allotted frequency channels to one or more neutral zones extending between the interference zones.
In yet another aspect of the present invention, there is provided a computer readable medium having stored thereon, computer-executable instructions which, when executed by a processor, cause the processor to perform the steps of: receiving input data including: (i) a desired carrier to interference ratio; (ii) the number of cells ixc3x97j in a fixed wireless network; and (iii) the number of frequency channels assigned to the network, wherein each of the cells includes a base station in radio frequency contact with a plurality of terminal stations located in the cell; calculating a reuse distance based on the desired carrier to interference ratio; calculating a cluster size based on the calculated reuse distance; determining the position and number of interference zones within each sector of the cells, wherein the interference zones are a function of the cluster size, and wherein each of the cells is divided into an even number of at least four sectors; assigning frequency channels to the interference zones; determining which of the allotted frequency channels cannot be used within a specified one of the cells; determining which of the allotted frequency channels cannot be used within a specified one of said sectors; and assigning any remaining ones of the allotted frequency channels to one or more neutral zones extending between the interference zones.